A Brighter Blue

Studies have suggested that colloidal nanocrystal quantum dots made up of II-VI compounds may be a cost-effective option for flat panel displays as well as for solid-state lighting. Most quantum dot LEDs, however, were developed as single-layer structures. Researchers from Pennsylvania State University in University Park and from Ocean NanoTech LLC of Fayetteville, Ark., used a multilayer structure to develop a quantum dot LED. Moreover, they demonstrated a blue-saturated quantum dot LED that is unique, according to assistant professor Jian Xu, because the long-wavelength (green and red) tail in the output of the LED is minimized to less than 5 percent of the total emission.

The electroluminescence spectrum of a blue quantum dot LED was measured at a bias of 5.5 V. The insets show photomicrographs at various brightness levels. Reprinted with permission of the American Chemical Society.Equally significant, according to Xu, is that their LED exhibits a maximum brightness of 1600 cd/m2 at just 5.5 V. The onset voltage (corresponding with a brightness of 0.1 cd/m2) was measured to be as low as 2.5 V. The low operating voltage and the high brightness achieved in these blue quantum dot LEDs became, for the first time, comparable to that of red and green quantum dot devices, thereby closing the performance gap between them.

The researchers’ test setup included a semiconductor parameter analyzer from Keithley Instruments Inc. of Cleveland, and they studied the LED output spectra with a spectrophotometer from Ocean Optics Inc. of Dunedin, Fla. To test the photometric brightness, they measured the diode’s output power with a silicon photodetector of known aperture size from Newport Corp. of Irvine, Calif. The diode output power was then converted to luminance. They recorded images of the output with a CCD camera from Sony Corp. of Tokyo.

Red, green and blue LED pixels with matched performance are necessary for the future development of flat panel displays and white-light sources based on quantum dots. Xu sees the result of this study as a further step toward the practical application of quantum dot LED technology in full-color displays and in solid-state lighting.

The next step is to work on the hole transport layer to increase the brightness. After that, the researchers plan to better engineer the quantum dot structure and to modify the device structure. They also hope to identify the origin of the emission by using time-resolved luminescence spectroscopy.